1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic system. In particular, the present invention relates to an image forming apparatus with an improved developing apparatus used in an image formation of an electrophotographic system.
2. Description of the Related Art
Development using a dry type toner is generally performed in a development process in an image forming process of the electrophotographic system. The toner is charged within a developing apparatus, and adheres to an image carrying body by electrostatic force to form a toner image.
The toner consists of an aggregate of fine particles having different particle diameters, but there are many cases where selective development by which toners of specific particle sizes are preferentially consumed, is performed in development. In particular, there are many cases where the selective development occurs in development processing of development is performed by applying alternating voltages.
In order to prevent the change of image quality caused by a change in particle size distribution of a developing agent due to the selective development, research have been made for measures regarding the selective development.
In Published Unexamined Japanese Patent Application No. 2001-242688 (hereinafter referred to as patent document 1), setting the frequency of an alternating bias voltage which forms an alternating electric field to a value at which the selective development does not occur is proposed.
In Published Unexamined Japanese Patent Application No. 2003-345136 (hereinafter referred to as patent document 2), selective development is prevented from occurring by devising the design of a magnet in a developing roller.
In Published Unexamined Japanese Patent Application No. 2004-333709 (hereinafter referred to as patent document 3), negative effect caused by the selective development is prevented by changing the direct-current voltage of a development bias when a developing apparatus is operated at non-image formation state, and by ejecting the toner, which has a reversed polarity from the developing apparatus to the image carrying body.
Moreover, in Published Unexamined Japanese Patent Application No. Hei 10-142908 (hereinafter referred to as patent document 4), preventing defective transfer from occurring by controlling the peak value of an AC component of a development bias voltage is proposed, though this prevention is not the prevention of selective development. That is, it is a developing apparatus which controls the peak value of an AC bias based on a history of a developing agent and prevents a defective transfer due to aged deterioration of the developing agent, therefore elongates usable period of the developing agent. The technique disclosed in patent document 4 cannot be used as a measure in a case where deterioration of a toner has progressed due to increase in the developing agent holding time when developing an image with a low printing rate.
In recent years, in a two-component developing system using a developing agent composed of a toner and a carrier, particle diameters of a toner which is easily used at time of development gets determined, and toner at the larger particle diameter side than the determined toner particle diameter or toner at the smaller particle diameter side than the determined toner particle diameter is used for development. Indefinitely, these toners stay in the developing apparatus. Consequently, problems concerning the progress of the deterioration of the toner, toner scattering, the deterioration of image quality, and the like are caused.
The phenomenon appears as a further remarkable phenomenon especially in the case where the printing rate is low. Moreover, when a developing agent stays in the developing apparatus and gets deteriorated, there is also a case in which the deterioration of the toner at the larger particle diameter side or at the smaller particle diameter side is large.
Accordingly, it became clear that the conventional techniques including the prior art mentioned above cannot prevent the selective development nor prevent the changes in image quality by the selective development sufficiently.
For example, by the technique disclosed in patent document 3, since the technique does not prevent any selective development while an image processing is conducted, image formation is performed by development accompanied with the selective development until ejection of toner is executed, and the deterioration of image quality cannot be fully prevented.
Moreover, the techniques disclosed in patent documents 1 and 2 adopt condition setting and apparatus configuration so as to prevent selective development. However, it is very difficult to adjust the setting or the apparatus configuration so as not to cause any selective development, and the changes in developing agents due to the selective development after a long period of operation and the changes of image quality is result are inevitable. Moreover, the deterioration of image quality due to the selective development does not only depend on the apparatus configuration and the particle size distribution of the toner used, but also depends on the characteristics of image to be formed and environment. Thus, the deterioration of the image quality due to the selective development cannot be fully prevented by an adjustment of the setting or the apparatus configuration.